Optical free-space transmission is subject to degradation arising from the effects of scintillation in the transmission medium. Free-space optical signals experience fading from scintillation over times on the order of several milliseconds. During these times, a multi-gigabit/second signal may lose tens of millions of bits. For example, an 8 millisecond fade in a 2.5 gigabit/second data stream equates to the loss of 20,000,000 bits.
When a data stream is transmitted over optical fibers, block-oriented forward error correction (FEC) is widely used. However, the degradation arising from scintillation effects in free-space optical transmission can last for millions of bits compared to the typically far shorter duration of error bursts in optical fiber media. The single-block FEC error correction process therefore is extremely impractical in correcting errors of the lengths that occur in free-space optical transmission. Addressing the problem resulting from scintillation effects by selective re-transmission of affected blocks is also not practical for such large error bursts, due to the real-time nature of communication processes such as video.
The practice of interleaving, or shuffling, data streams is used to achieve greater robustness and noise reduction in many communications applications. The general principle, as stated in the publication “Error Correcting Codes”, W. Peterson and E. J. Weldon, MIT Press (1972), p. 371, (which is hereby incorporated by reference) is that a t-error-correcting code interleaved to degree i is capable of correcting all single bursts of length i*t or less. Through interleaving, the burst error can be made to have the effect of many isolated errors, provided that the data is interleaved over a span large compared to the burst duration.
Interleaving is therefore a candidate for error-correction of scintillation effects in optical free-space transmission, since in theory it can correct for error bursts that last for tens of millions of consecutive bits. However, in correcting tens of millions of bits in a burst error within a single code block, the communication terminal requires an extraordinarily large encoder/decoder and buffer store under current terminal design concepts. Considerations of cost, size, and power consumed by the scintillation effect error-correction apparatus must be constrained in realizing a commercially practical communication terminal.
Synchronous-dynamic-random-access-memory (SDRAM) devices are available that can provide a practical means of realizing the large permutation matrix capacity required to interleave. However, the SDRAM rows, or “pages”, typically incur a significant, multi-cycle overhead cost whenever it is necessary to change the row address, i.e. change “pages”. The column address field typically equates to the low order digits in the physical address of the SDRAM, such that when addressing consecutive or grouped sets of data, a minimum of page changes is incurred. If the SDRAM devices were addressed in a straightforward way, namely storing segments of the block FEC codeword in successive addresses, then reading-out of the codeword sequence in permuted order to effect the interleaving requires changing page addresses each time the memory is referenced, due to the large address increments required. An analogous process is that of storing a matrix with row elements in successive addresses, then reading the matrix by column. Address increments equal to the row length would be required in reading the transposed matrix.
More specifically, in a suitable SDRAM device grouping arranged to provide buffer store for the large permutation matrix needed for interleaving and de-interleaving, a large number of cycles (7 for example) are required to change page addresses. The result of straightforward addressing then is a seven-fold reduction of the effective memory speed. The high bit rates involved in optical communications already mandate using the fastest SDRAM devices available to perform this interleaving process. Therefore, such a slowdown would impair the practicality of using SDRAM devices to implement a process of overcoming scintillation effects in optical free-space transmission.